1. Field of the Invention
This relates to lasers and more particularly to passively mode-locked lasers useful as sources of stable short pulses.
2. Description of the Prior Art
As is known, a passively mode-locked laser is a free-running laser which produces a periodic series of pulses whose repetition rate is related to the time it takes a pulse to make a round trip between the two ends of the laser cavity. To produce mode locking passively, a saturable absorber is also included wihin the resonant cavity. A particular form of passively mode-locked laser, pupular because it can be tuned to provide a desired wavelength over a relatively broad band of wavelengths, utilizes an optically pumped "dye" cell as the emitting element and a dye medium as the saturable absorber. An example of a mode-locked laser of this kind is described in our paper published in Applied Physics Letters Vol. 24 No. 8, 15 Apr. 1974 pps. 373-375 entitled "Subpicosecond Kilowatt Pulses from a Mode-Locked Dye Laser".
This laser is of particular interest since it is capable of providing pulses of very short duration, such as a picosecond or less.
However, in such lasers, to achieve short pulses it has been found necessary to operate near the threshold of laser operation and, because of the near-threshold condition, to stabilize all parameters, such as mirror positions and pumping power, to a high degree. These requirements prove difficult to maintain in practice and as a result, when the laser is operated to provide very short pulses such as picosecond pulses, the length of the pulses tends to be unstable.
Accordingly, the invention is directed at improving the stability of passively mode-locked lasers, particularly such lasers designed to provide very short pulses.